The present invention relates to a nozzle for a gas-insulated switching device for high- or medium-voltage applications, i.e. for voltages above 1000 Volt, and to a related switching device comprising such a nozzle.
As it is widely known in the art, gas insulated switching devices for high- and medium-voltage applications normally comprise a fixed arc contact and a mobile arc contact; it is also known that several types of gas switching devices for high- and medium-voltage applications may additionally have a so-called fixed main or permanent contact and a corresponding mobile main or permanent contact through which the current mainly passes when the gas switching device is in a closed position.
In both solutions, during opening/closing operations, usually electric arcs strike between the arcing contacts; a nozzle assembly is generally positioned around the arc contacts in order to delimit the arcing zone and to restrain the potentially negative effects of such electric arcs. By means of the nozzle assembly, a sufficient flow of gas is obtained in the arcing zone so as to cool the arcing contacts sufficiently and remove heat; thus, circuit breaking is enabled to be performed over the entire required interruption window and for currents going up to the short-circuit breaking capacity.
Examples of existing nozzles are disclosed in patent documents U.S. Pat. No. 6,483,064, U.S. Pat. No. 5,216,214 and EP 0524088.
At present, known nozzle assemblies have some drawbacks, as regard in particular to their constructive structure, manufacturing, and mounting in switching devices as well.
Indeed, known nozzle assemblies are generally realized by means of several different pieces, mainly a converging-diverging body, made of insulating material, a so-called puffer cap, and other additional components for realizing the mechanical connections. In particular, the insulating body is generally realized by using a cylinder of raw material which is machined, e.g. with a lathe, so as to obtain the desired shape; alternatively, the raw material is first preformed, thus achieving a rough shape, and then machined with suitable tools. In both cases a lot of raw materials is wasted and machining is usually difficult and time consuming.
Hence, costs and manufacturing time are not optimized, whilst assembly inside the switching device, due to the use of different pieces, is rather cumbersome and mechanically complicated. In particular, when the main contacts are present, the nozzle assembly is mounted by means of mechanical connection means on the mobile equipment of the main mobile contact, as described, for example in U.S. Pat. No. 5,939,692; in addition to being more complicated, this solution is clearly not suitable when the switching device has only the arc contacts, thus forcing to adopt different constructive solutions and lacking in flexibility. Finally, gas switching devices for high- and medium-voltage applications may further comprise an electrically conductive shield, positioned externally to and operatively associated with the nozzle assembly, in order to optimize the electric field distribution in the arcing zone and in its surroundings. Therefore, also this function is usually performed by means of an additional dedicated component and results in a non-optimized space occupation inside the device.
Alternatively, when the main contacts are present, the mobile main contact is machined so as it has a shape which performs the shielding function; in this case, particular care must be taken when shaping the contacts and assembling them on the structure of the switching device in order to avoid damages. This clearly results in a strong complication in the machinery of the mobile main contact and in assembling the switching device, with a consequent increase in manufacturing time and cost.